Magnetic flowmeters adapted to measure volumetric flow rates of conductive fluids by capacitive signal pickup means have been devised for some time. In such meters the electrodes are electrically insulated from the fluid to be measured by a dielectric liner. Thus a capacitor is formed between the conductive fluid and each of the electrodes. The advantages of using "non-wetted" electrodes in handling troublesome process fluids in a capacitance type instrument (i.e., corrosive fluids, "dirty" fluids which tend to coat or foul the electrodes, and low conductivity fluids, to name a few) are also well known to those of skill in the art.
Examples of prior patent art in this area include U.S. Pat. No. 3,839,912 which discloses a magnetic flowmeter system with measuring electrodes that are capacitively coupled to the fluid. The flow tube of the system is adapted to receive an insertable probe section formed of dielectric material having an integral electrode assembly. Positioned within this assembly and connected to the measuring electrode is a high-impedance amplifier for receiving the flow induced voltages. These flow related signals result from the interaction of an a-c magnetic field established across the flow tube and the flowing conductive fluid that induces an a-c voltage at the electrodes through the capacitive coupling with the fluid. The input impedance of the amplifier is kept extremely high so that the flow induced voltage may be suitably amplified by the amplifier to provide an output signal substantially proportional to the flow rate.
U.S. Pat. No. 3,999,443 discloses another variation of a capacitance type magnetic flowmeter system wherein the measuring electrodes are imbedded within the dielectric liner of a flow tube mounted within the flow stream. This disclosure is primarily concerned with minimizing spurious voltages produced during instrument operation, i.e., stray capacitance-coupled voltages and induced loop voltages in the input leads. As taught in this patent, spurious voltages from the first source are minimized by a combination of electrostatic shielding and low-frequency excitation of the magnetic field, while induced loop voltages are taken into account by varying the magnetic field in accordance with a square-wave excitation. Thus according to the disclosure a period exists in the measurement cycle where the rate of change of the magnetic field is zero thereby minimizing undesired in-phase and quadrature voltage components. Just as with the aforementioned U.S. Pat. No. 3,839,912, this patent teaches that in order to obtain an output of desired accuracy the electrodes are connected to a high-impedance amplifier.
Because the disclosures of the prior art are all concerned with processing an extremely high-impedance measurement voltage at the electrodes, with corresponding electrode connection to a high-impedance amplifier circuit, certain signal handling difficulties are encountered. For example, such high-impedance signals are sensitive to cable characteristics and stray capacitance thereby requiring special attention to be paid to shielding. Furthermore, magnetic flowmeter systems that utilize high-impedance measurement signals generally involve high-frequency/high-power operation to produce an output indicative of flow rate. Aside from being energy inefficient, this mode of operation involves the use of critically-tuned circuits with attendant increase in complexity and cost.
Attempts to overcome certain of these difficulties can produce other difficulties. For example, as discussed in the aforementioned U.S. Pat. No. 3,999,443, spurious voltages produced by stray capacitance leakeage may be minimized in part by low-frequency excitation of the flowmeter drive system. However, this has the effect of further increasing the capacitive impedance between the electrodes and the process fluid resulting in further power drain and an actual increase in susceptibility to stray leakage.